Early detection of polyps in a colon, which are typically considered precursors to cancer, can greatly increase a patient's chance of survival from colon cancer. Regrettably, many patients are discouraged from obtaining conventional colonoscopies because of the discomfort and invasiveness of the procedure. A “virtual colonoscopy” avoids some of the invasiveness of a conventional colonoscopy and can increase the likelihood of early detection of colon cancer.
In general, a “virtual endoscopy” is performed with a “flythrough” of a computer generated three-dimensional image of an anatomical structure. These methods use computed tomographic (CT) scans and computer imaging software. Three-dimensional modeling has been used in a variety of clinical applications including virtual colonoscopies, virtual bronchoscopies, and virtual angioscopies. In a typical flythrough, a user can move a viewpoint through the reconstructed volumetric object, stopping at certain points to further analyze a suspicious formation on the inner wall of the anatomical model. Virtual colonoscopies have been shown to be effective at detecting small polyps. However, when only one scan is used, extraneous material may be present and cause interference.
Extraneous material such as stool and water can cause both false positives and false negatives. Because the texture and color of a 3D colon model are not represented, the presence of extraneous material makes identification of a polyp-shaped formation more difficult. These types of extraneous materials can obscure a true polyp, causing a false negative. Alternatively, the extraneous material could be mistaken for a polyp, in which case a practitioner may identify a pseudo-polyp and declare a false positive. Using two scans, for example a prone and supine scan, can avoid these problems. Because extraneous materials often shift position between the prone and supine scans, a practitioner can use both scans to detect and differentiate between a true polyp and a pseudo-polyp. The practitioner may also be able to detect polyps that were previously obscured by material in one scan.
During a flythrough, a practitioner may choose to change views from one scan to the other to further analyze a portion of the interior of the virtual colon. After the view change, for the sake of efficiency, it would be ideal to place the practitioner's viewpoint at the same position in the colon. However, typically, the difference in shape and size of the colon between the prone and supine scans can be relatively large. This makes it difficult to manually determine the corresponding position between the scans.
Generally, registration is a method of determining a set of corresponding points between two or more scans. One method of registration is manual registration performed by the practitioner. Typically, the practitioner views the scans together and attempts to pick out characteristic anatomical landmarks to create a baseline correspondence. After this initial orientation, when a practitioner finds a suspicious formation, he can then orient himself in a corresponding scan using the baseline correspondence and page through adjacent images until he is near the same location. This method is tedious, inaccurate, and costly.